1. The Field of the Invention
The present invention relates to the deposition of mercury (Hg) into defined locations and to devices for such dispensing. More particularly, the present invention includes mercury-dispensing devices for the introduction of mercury into electron tubes.
2. The Relevant Art
The use of small amounts of mercury in devices such as electron tubes, for example, mercury-arc rectifiers, lasers, various kinds of alphanumeric displays and, particularly, fluorescent lamps, is well known. Providing the minimum quantity of mercury required inside these devices is extremely important to maintain the performance of these devices, and, especially, to minimize environmental impact during their construction and use. The high toxicity of mercury also poses serious ecological hazard relating to the disposal of mercury-containing devices. Such concerns have been the subject of legislative focus, and recent international regulations have sought to establish upper limits for the amount of mercury that can be used in these devices. For example, it has been suggested that fluorescent lamps include no more than 10 milligrams (mg) of mercury per lamp.
Mercury has been introduced into electron tubes in liquid form. However, the high vapor pressure of mercury at room temperature posed problems for its storage and handling. Also, introducing precise and reproducible doses of microliter quantities of liquid mercury was extremely difficult to control, and often resulted in the introduction of excess amounts of the element into the device.
The use of liquid mercury contained in capsules has also been disclosed, for example, in U.S. Pat. Nos. 4,823,047 and 4,754,193, referring to the use of metallic capsules, and in U.S. Pat. Nos. 4,182,971 and 4,278,908 wherein the mercury container is made of glass. After introducing the mercury-containing container into the electron tube, the mercury is released by means of a heat treatment which causes the container tip to break.
These methods generally have several drawbacks. First, the production of the capsules and their mounting inside the electron tubes is complex, especially where the tubes are small. Second, breaking a capsule, especially a glass capsule, can produce fragments of material that can impair the functioning of the electron tube. To address the latter problem, U.S. Pat. No. 4,335,326 discloses an assembly wherein the mercury-containing capsule is itself located inside a capsule which acts as a shield for the fragments. Third, the release of the mercury is often violent and may damage the inner structure of the tube. Finally, capsule systems still use liquid mercury, and therefore do not completely solve the problems of delivering precise and reproducible amounts of a few milligrams of mercury into a small space.
U.S. Pat. No. 4,808,136 and European Patent Application Serial No. EP-568,317 disclose the use of tablets or small spheres of porous material soaked with mercury which is released by heating once the tube is closed. However, these methods also require complicated operations to loading the mercury into the tablets, and the amount of mercury released into the tube is difficult to control reproducibly. In addition, these methods still involve liquid mercury.
The use of amalgams of mercury with, for example, indium, bismuth, or zinc, is also known. In general, however, these amalgams have the drawback of a low melting point coupled with high mercury vapor pressure at relatively low temperatures. For example, the zinc amalgams described in the commercial bulletins of APL Engineering Materials Inc., have a mercury vapor pressure at 43.degree. C. which is about 90% of that of liquid mercury. Consequently, the amalgams do not easily withstand the thermal treatments employed in the production of the electron tubes into which the amalgams are introduced, during which treatments the mercury-dispensing devices may reach temperatures of about 400.degree. C.
These drawbacks are overcome in U.S. Pat. No. 3,657,589, which discloses the use of intermetallic compounds of titanium (Ti), zirconium (Zr) and mercury having the general formula Ti.sub.x Zr.sub.y Hg.sub.z, in which x and y may vary between 0 and 13, the sum x+y may vary between 3 and 13, and z may be 1 or 2. These compounds have mercury-release temperatures which vary according to the specific composition of the intermetallic compound. However, all of these compounds are stable up to about 500.degree. C., both in the atmosphere and in vacuo, making them compatible with the assembly operations for electron tubes. The mercury is released from the above-cited compounds by an activation operation, which is usually carried out by heating the material between 750.degree. C. and 900.degree. C. for about 30 seconds. This heating may be accomplished by laser radiation, or by induction heating of the metallic support of the mercury-dispensing compound. The use of the Ti.sub.3 Hg compound (x=3, y=0 and z=1), manufactured and sold by SAES Getters S.p.A. (Milan, Italy) under the trade name St 505, has been shown to be particularly advantageous because of its availability in the form of a powder compressed in a ring-shaped container or in pills or tablets, sold under the trademark "STAHGSORB", or in the form of powders laminated on a metallic strip, sold under the trademark "GEMEDIS".
In addition to the above-described stability during the production cycle of the tubes, during which temperatures of about 350-400.degree. C. may be reached, the Ti.sub.x Zr.sub.y Hg.sub.z compounds can also be combined with a getter material can be easily added to the mercury-dispensing compound for the purpose of chemisorption of gases such as carbon monoxide (CO), carbon dioxide (CO.sub.2), molecular oxygen (O.sub.2), molecular hydrogen (H.sub.2) and water (H.sub.2 O), which would interfere with the tube operation; the getter is activated during the same heat treatment in which the mercury is released as described in U.S. Pat. No. 3,657,589. Furthermore, the amount of mercury released by the Ti.sub.x Zr.sub.y Hg.sub.z compounds is controllable and reproducible.
Despite their good chemical and physical characteristics, and their ease of use, these materials have the drawback that the contained mercury is not completely released during the activation treatment. Furthermore, production processes for mercury-containing electron tubes include a tube closing operation performed by either glass fusion, e.g., for the sealing of fluorescent lamps, or by frit sealing, e.g., in welding two pre-shaped glass members by means of a paste of low-melting glass. During these operations, the mercury-dispensing device may undergo an indirect heating up to about 350-400.degree. C. In this step, the dispensing device is exposed to gases and vapors emitted by the melted glass and, in almost all industrial processes, to air. Under these conditions, the mercury-dispensing material undergoes a surface oxidation, which results in a yield (i.e., the percentage of mercury which is released) of about 40% of the total mercury content during the activation process. The mercury not released during the activation operation is then slowly released during the life of the electron tube. This characteristic, together with the fact that the tube must obviously work from the beginning of its life cycle, leads to the necessity of introducing into the device about twice as much mercury than that which would be theoretically necessary.
In order to overcome these problems, European Patent Application Serial No. EP-A-091,297 suggests the addition of nickel (Ni) or copper (Cu) powders to the Ti.sub.x Zr.sub.y Hg.sub.z compounds in which x=3, y=0 and z=1 (Ti.sub.3 Hg) or x=0, y=3 and z=1 (Zr.sub.3 Hg). According to this document, the addition of Ni or Cu to the mercury-dispensing compounds causes melting of the mercury-containing materials, favoring the release of almost all of the mercury in a few seconds. The melting takes place at the eutectic temperatures of the Ni--Ti, Ni--Zr, Cu--Ti and Cu--Zr systems, ranging from about 880.degree. C. for the Cu 66%-Ti 34% composition to about 1280.degree. C. for the Ni 81%-Ti 19% composition (atomic percent). However, the document erroneously gives a melting temperature of 770.degree. C. for the Ni 4%-Ti 96% composition.
Despite the advantages disclosed in EP-A-091,297, this document acknowledges that the mercury-containing compounds disclosed therein undergo chemical changes during the tube working treatments, and thus need protection. To this end it is suggested to enclose the mercury-containing material in containers made of a steel, copper or nickel sheet which is broken during the activation by the pressure of the mercury vapor generated inside the container. This solution is not completely satisfactory, however. As described above with respect to the capsule mercury dispensers the mercury bursts out of the containers violently, possibly damaging portions of the electron tube. Also, manufacturing such containers is quite complicated, requiring the welding of small metallic parts.
Thus, it would be advantageous to provide a mercury dispenser that is capable of delivering small amounts of mercury into devices such as electron tubes reliably, controllably, reproducibly and with little or no damage to other components in the device.